The present invention relates to an image forming apparatus incorporating a developing device for use with powders and having a multicolor image editing function.
Developing units are in wide use which comprise a developing sleeve having incorporated therein a plurality of magnets extending axially thereof, and a bristle height restricting member opposed to and spaced from the outer peripheral surface of the developing sleeve. A magnetic developer is supplied to the outer peripheral surface of the developing sleeve and transported in the form of a magnetic brush, with the amount of transport controlled by the restricting member.
On the other hand, copying machines having various functions have been proposed in recent years. These copying machines include twin color copying machines which have two developing units of the above type provided around a photosensitive drum and containing developers of different colors. The color copying machine has a two-color image editing function, such that a portion of a document image is developed in one of the colors and the other portion thereof in the other color.
However, when the above function is to be performed, there is a need to execute twice an image forming process comprising the sequence of steps of charging, exposure, local erasure, development, transfer and fixing and to provide a return path of transport through which copy paper having an image formed thereon by the first cycle of the process is guided to the second cycle of the process. This entails the problem that the apparatus becomes large-sized, complex in construction and costly.
Another problem is also encountered. Since the copy paper is subjected to the image forming process twice in succession and therefore to mechanical and thermal stresses, the paper is prone to curling and damage, consequently jamming the apparatus or causing displacement of the image.
The apparatus has another problem in that it requires for copying approximately twice the time needed for single-color copying and is extremely low in processing efficiency per unit time.